1. Field of the Invention
The present invention relates to a process for ion nitriding aluminum or aluminum alloys.
2. Description of the Prior Art
Since aluminum and aluminum alloys (hereinafter referred to as aluminum material) have low hardness and poor wear resistance, attemps have been made to develop surface treating methods for improving these properties. However, aluminum material has strong affinity to oxygen in the air and combines readily with oxygen to form a stable, dense and thin layer of alumina (Al.sub.2 O.sub.3) thereon. Therefore, the surface treating method for aluminum material has limitations, as compared with surface treatment of iron or ferrous alloys, and only such surface treatment as formation of an alumina coating film by anodic oxidation has been put into practice. However, the alumina coating film merely has a Vickers hardness of about 200 to 600 (variable with the treating conditions) and thus it has not sufficient wear resistance.
On the other hand, as a coating film having higher hardness than that of the alumina coating film, there is an aluminum nitride (AlN) coating film. Aluminum nitride is useful since it is stable up to a very high temperature of 2000.degree. C. or above and has excellent wear resistance, high thermal conductivity and good insulating properties.
Aluminum has strong affinity to nitrogen and combines readily with nitrogen to form aluminum nitride. Therefore, attempts have been made for forming aluminum nitride on the surface of aluminum material. For example, there are a melting method in which a part of aluminum material as a material to be treated is melted and nitrided, a reactive sputtering or reactive vapor deposition method, and the like. However, in the melting method, the material to be treated is deformed through melting and the obtained aluminum nitride layer has a Vickers hardness as low as 200 or less. Further, the reactive sputtering or vapor deposition method has drawbacks, such as poor adhesion between the aluminum nitride layer and the material to be treated, difficulty in treating many articles and high treating cost.
For realizing a method not using the melting method and enabling the treatment of many aluminum articles, there was an attempt to apply an ion nitriding method for treating iron or ferrous alloys to the formation of an aluminum nitride coating film having excellent wear resistance. However, such attempt has been found difficult because of an alumina layer easily formed on an aluminum article to be treated as mentioned above.
A nitriding treatment for aluminum articles of a plate-shaped or rod-shaped form has not been possible because aluminum material easily reacts with oxygen to form an alumina (Al.sub.2 O.sub.3) layer thereon before nitriding as mentioned above. It has only been possible to obtain AlN powder by heating aluminum or aluminum alloy powder in a nitrogen or ammonia atmosphere. However, this method requires much expense and time. Further, it cannot be applied to direct nitriding treatment of aluminum articles having a plate-shaped or rod-shaped form.